1. Field of the Invention
This invention relates to a variable-capacitance diode element, and more particularly it pertains to such a diode element which is improved in respect of saturation tendency of the capacitance-voltage characteristics thereof and has the high-frequency serial resistance R.sub.s reduced so as to represent an enhanced quality factor Q.
2. Description of the Prior Art
Generally, most variable-capacitance diode elements are fabricated in the form of planar construction. Description will now be made of a conventional variable-capacitance diode element with reference to a fabricating process as illustrated in FIGS. 1(a) to (c).
In FIG. 1, an N-type semiconductor substrate 1 having a lower resistivity is prepared which is provided, by means of a vapor-phase growth process, with an N-type epitaxial layer 2 having a higher resistivity than that of the semiconductor substrate 1, say about 1 .OMEGA.cm and a thickness of about 4-5 .mu.m (FIG. 1(a)). A major surface of the epitaxial layer 2 is subjected to a thermal oxidization treatment so that a thermally oxidized film (SiO.sub.2) 3 is formed in a thickness of about 1 .mu.m, and then an opening portion 6 is formed in the film 3 by means of an etching process. Subsequently, by means of an ion implantation process and under such conditions as acceleration voltage of 130 KeV and dosage of (2 to 3).times.10.sup.13 cm.sup.-2, an N-type impurity element is implanted into the the epitaxial layer 2 through the opening portion 6 through which the major surface of the epitaxial layer 2 is exposed, thereby forming an N-type diffusion layer. It is also possible that the ion implantation may be effected in such a manner that the impurity element is implanted through an oxide film of 100-3000 .ANG.. Thereafter, the resultant composite structure is subjected to a heat treatment which also serves to effect annealing for recovery of latice defects resulting from the ion implantation and carrier recovery, thus resulting in an N.sup.+ -type diffusion layer 4 being formed which has a higher impurity concentration than that of the aforementioned epitaxial layer (FIG. 1(b)). Subsequently, a P.sup.++ -type diffusion layer 5 is formed which has a smaller diffusion length than that of the diffusion layer 4 and covers the exposed portion of the diffusion layer 4 in such a manner that a PN junction is defined between the diffusion layer 5 and the diffusion layer 4 and epitaxial layer 2 (FIG. 1(c)). Electrodes are then provided at the top and bottom of the resulting composite structure respectively, and in this way, a variable-capacitance diode element is provided.
With reference to FIG. 2, explanation will be made of depletion layer of variable-capacitance diode element. Assuming that the impurity concentration of the P.sup.++ -type diffusion layer 5 is sufficiently higher than that of the N.sup.+ -type diffusion layer 4 and epitaxial layer 2, the width of depletion layer extending in the P.sup.++ -type diffusion layer 5, when a reverse bias voltage V.sub.R is applied, becomes very small, i.e., negligible as compared with the width of depletion layer extending in the N.sup.+ -type diffusion layer 4 and epitaxial layer 2. The variable capacitance C.sub.j of the variable-capacitance diode element is considered to consist of a combination of a junction capacitance C.sub.j1 due to a depletion layer 7 resulting from the PN junction between the N.sup.+ -type diffusion layer 4 and the P.sup.++ -type diffusion layer 5 and a junction capacitance C.sub.j2 due to a depletion layer 8 resulting from the PN junction between the epitaxial layer 2 and the diffusion layer 5. The impurity concentration of the epitaxial layer 2 is lower than that of the N.sup.+ -type diffusion layer 4 so that the width of the depletion layer 8 of the epitaxial layer becomes greater than that of the depletion layer 7 of the diffusion layer 4. Furthermore, by increasing or decreasing the applied voltage V.sub.R, the width of the depletion layer 7, 8 is increased or decreased so that the capacitance C.sub.j1, C.sub.j2 is varied, thus resulting in variable capacitance C.sub.j which consists of a combination of the capacitances C.sub.j1 and C.sub.j2 as mentioned above.
The variable capacitance of the variable-capacitance diode element is given by the following expressions: ##EQU1## where W.sub.j is the width of depletion layer; N (x) is the impurity concentration; K.sub.S is the dielectric constant of the semiconductor substrate; .epsilon..sub.O is the dielectric constant in a vacuum (8.85.times.10.sup.-12 F/m.sup.2); q is electron charge (1.60.times.10.sup.-19 C); .PHI..sub.B is the diffusion potential at the PN junction; n is a constant determined from the concentration gradient of the impurity element in the diode element; and A is the area of the diode element.
It will be seen from Equations (1) and (2) that the width of depletion layer depends on the impurity concentration of the semiconductor layer forming the PN junction with the P-type diffusion layer 5. Making an explanation with reference to FIG. 3, when reverse bias voltage V.sub.R is applied to the variable capacitance diode element, the width W.sub.j2 of the depletion layer 8 in the epitaxial layer 2 the impurity concentration of which is lower than that of the diffusion layer, becomes greater than the width W.sub.j1 of the depletion layer 7 in the diffusion layer 4; and as the applied voltage V.sub.R is increased, the depletion layer 8 is caused to extend until it contacts the semiconductor substrate 1, and further extension thereof is prevented.
A further explanation will be made with reference to the capacitance-voltage characteristics illustrated in FIG. 3. With the conventional variable-capacitance diode element, as shown by curve (I), if the applied voltage exceeds V.sub.O, then the slope of the curve will become gentler so that saturation tendency be occur and thus the capacitance will be saturated at C.sub.S. Such conventional variable-capacitance diode element is disadvantageous in that the depletion layer 8 extending from the periphery of the diffusion layer 4 is caused to extend into contact with the semiconductor substrate 1 and the rate of change of the capacitance C.sub.j with voltage becomes lower. Thus, it has so far been a task to increase the quality factor Q and improve the saturation tendency of the capacitance-voltage characteristics without increasing the high-frequency serial resistance R.sub.S.
By increasing the thickness t of the epitaxail layer 2, it is possible to prevent the depletion layer 8 from extending into contact with the semiconductor substrate 1, thereby solving the aforementioned problem. However, such a procedure is disadvantageous in that the increase in the thickness of the epitaxial layer 2 immediately below the diffusion layer 4 results in the high-frequency serial resistance R.sub.S being increased so that the quality factor Q is decreased. Thus, it is not allowable to make the epitaxial layer 2 thicker than in the prior art.